1. Field of the Invention
The present invention relates to methods of treating paper making equipment, including felts and fabrics, to eliminate or reduce the need to shut-down and clean such equipment. More specifically the invention relates to continuous or intermittent treatment of papermaking equipment with a composition containing at least one nonionic surfactant that contains units of ethylene oxide (EO) and of a higher alkylene oxide (AO), alone or in combination with an anionic dispersant or sequestering agent to inhibit deposition.
2. Discussion and Background
Paper is produced in a continuous manner from a fibrous suspension (pulp furnish) generally made of water and cellulose fibers. A typical paper manufacturing process consists of 3 stages: forming, pressing, and drying. In the forming stage, the dilute pulp furnish is directed on a forming fabric or between two fabrics. The majority of the water is drained from the pulp furnish, through the fabric, creating a wet paper web. In the pressing stage the paper web comes in contact with one or generally more porous press felts that are used to extract much of the remaining water from the web. Often the pickup felt is the first felt that the wet web contacts which is used to remove the web from the forming fabric, via a suction pickup roll positioned behind the felt, and then to transport the web to the rest of the press section. The web then generally passes through one or more presses each consisting of two press elements such as rotating press rolls and/or stationary elements such as extended nip or shoe presses. The two press elements are positioned in close proximity to each other forming, what is commonly referred to as, a press nip. In each nip the paper web comes in contact with either one or two press felts where water is forced from the web and into the press felt via pressure and/or vacuum. In single-felted press nips the web is in contact with the press roll on one side and the felt on the other. In double-felted press nips, the web passes between the two felts. After the press section, the paper web is dried to remove the remaining water, usually by weaving through a series of steam heated dryer cans.
Press felts are normally an endless loop that circulates continuously in a belt-like fashion between sheet contact stages and return stages. Various felt carrier rolls are used to keep the felt loop rotating through these stages. Water pulled into the felt from the paper web at the nip is generally removed from the felt by vacuum during the felt return stage at, what is frequently referred to as, the uhle box. The uhle box is also sometimes called the felt suction pipe, or the suction box. The uhle box cover is commonly made of materials such as ceramic or high molecular weight polyethylene with openings often in slots or herringbone patterns through which vacuum is applied to the felt. Generally lubrication shower water is applied to the felt just prior to where the felt contacts the uhle box to reduce friction on the felt and cover surfaces.
Press felts often consist of nylon base fabric generally made of from 1 to 4 individual layers of filaments arranged in a weave pattern. An extruded polymeric membrane or mesh can also be included as one or more of the base fabric layers. Batt fibers, of smaller diameter than the base fabric filaments, are needled into the base on both sides giving the felt a thick, blanket-like appearance. Press felts are designed to quickly take in water from the web in the nip and hold the water so that it does not re-absorb back into the sheet as the paper and felt exit the press nip. Press felts are designed to efficiently remove water from the paper web, however, the press felt design also tends to be a trap for materials that are dissolved or suspended in the liquid coming from the paper web, causing these materials to build-up on or within the felt structure.
It is common for materials that build-up in the press felt to transfer from the felt to other press section surfaces and to form deposits on these surfaces. For example uhle box deposition is a problem that is often associated with newsprint manufacture. Newsprint is made from pulp that is high in resinous/sticky contaminants. The resins tend to transfer from the felt to the uhle cover, where deposits build to a point where the uhle box can no longer pull a vacuum on the felt. Such resin build-up forces the paper maker to routinely shut-down the paper machine to clean the deposit from the cover. Other equipment surfaces within the press section where problematic deposits can build include press rolls, doctor blades used to clean press rolls, felt carrier rolls, and press belts. Press belts are used on stationary types of presses (shoe and extended nip) such that the belt separates the felt from the stationary element.
It is important for efficient paper production, that press felts remain deposit-free. Deposits that form on press felts such as oily or sticky materials can transfer back to the web resulting in dirt spots or holes in the finished paper. They can also cause paper breaks or tears leading to lost production. It is also important for efficient paper production, that press felts remain porous with high void volume. It is highly expensive and energy intensive to evaporate water from paper in the dryer section, making it critical that the press felts remove as much water as possible from the paper web in the press section. Felts that become filled with contaminants that limit water movement through the felt will thus limit the amount of water that can be removed from the web. This will force the machine speed to be slowed in order to allow time for the web to dry in the dryer section. Felts that are unevenly filled can also lead to uneven water removal from the sheet which can result in moisture streaks, wrinkles, and web breaks.
It is also highly important for efficient paper production, that the other equipment surfaces within the press section remain free of deposits. In particular deposits that form on the uhle box covers can impede water removal from the felt and thus limit water removal by the felt from the paper web. Deposits can also build to a point where they break off and transfer back to the felt and to the web leading to web breaks and lost production. Deposition on the uhle cover, carrier rolls, and press rolls can also lead to excessive wear of the nylon fibers in the felt resulting in premature removal. Press section deposition can cause down time necessary for cleaning resulting in significant losses in production time.
Some of the dissolved or suspended materials that are present in the web that can deposit in the press section include components originating from pulp such as wood pitch including resin acids, fatty acids, and fatty esters and stickies from recycled pulp including inks, glues, resins, latexes, and waxes. Carryover of processing additives used in the pulp mill can also lead to press section deposition such as sodium silicates commonly added during mechanical pulp and deinked pulp bleaching; and carryover of deink plant flotation collectors. Cellulose fines and hemicelluloses can also add bulk to the deposits. Byproducts of microbiological growth such as polysaccharides, proteins, and other biological matter, can deposit in the press section. Various functional additives that are added to paper stock to impart certain properties to the finished paper can deposit in the press section. These additives include sizes such as rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA); wet strength resins and dry strength agents; and inorganic fillers including clay, talc, precipitated or ground calcium carbonate (PCC, GCC), and titanium dioxide. Processing additives used to improve or limit problems during paper production that can deposit in the press section include retention and drainage aids such as alum, organic polymers, and various micro-particles; and defoamers, in particular those based on oil.
It is well known that felt conditioners enhance the performance and extend the effective life of felts by inhibiting the materials mentioned above from filling the void volume in press felts. Felt conditioners are usually liquid blends of surfactants, dispersants and/or polymers most often in water but other solvents are also utilized. Oxidizers, acids, and alkalis can also be contained in felt conditioners, generally in relatively low concentrations. Felt conditioners are applied continuously or intermittently to papermaking felts while paper is being produced through showers during the fabric return stage, while the felt is not in contact with the paper web. These treatments are most often applied on the inside, or machine side, of the felt through low pressure showers, often just prior to a felt carrier roll such that hydraulic force will help move the chemical into the felt to help prevent and remove contaminants that fill the felt. Such treatments are also sometimes applied, through similar showers on the sheet side of the felt after the uhle box and before the nip so that the treatment is present on the surface when contaminants first reach the felt. Additional water showers that are commonly used on press felts and in which chemicals could be applied include high pressure showers that are usually employed intermittently, so as not to damage the felt, and are most often used on the sheet side to remove surface contaminants. Lubrication showers are also commonly used to apply water at the entrance to the uhle box to prevent wear and provide a seal so that vacuum can remove fluid from within the felt.
When the felts become too filled that they no longer allow for efficient paper manufacture, it becomes necessary to clean them by a process commonly referred to as batch cleaning. When felts are batch cleaned, paper production is stopped, the felt speed is generally slowed, the vacuum at the uhle box is stopped or significantly reduced, and showers are turned off with the exception of the chemical shower. A cleaning solution, generally consisting of high concentrations of caustic, acid, solvent such as kerosene, and/or oxidizer such as hypochlorite, is applied through the chemical shower. After sufficient time for the cleaning solutions to penetrate the filling material, water showers are employed such that the contaminants and batch cleaning chemicals are removed from the felt by vacuum at the uhle box. It is generally necessary to remove the batch cleaning chemicals from the press felt because these materials, at the high concentrations utilized, can damage the press felt if allowed to remain on the felt or can transfer back to the paper altering its characteristics. In some instances it may be necessary to batch clean felts multiple times in a 24-hour production day. Batch cleaning is often necessary, but not a desirable solution since the chemicals used are often hazardous, environmentally unfriendly, and can damage the felt with repeated use. Valuable production time is lost during shut-downs for batch cleaning. If such cleaning is unsuccessful, it is necessary to remove the felt, sometimes prematurely, from the paper machine, which is costly from both a time and material perspective.
Deposits that form on press section equipment are not always prevented effectively by felt conditioners. Felt batch cleaners, used to clean the felt, do not always adequately remove the deposits formed on other press section surfaces, and the felt may not always need to be cleaned when other press section surfaces need to be cleaned. It is often necessary to shut-down the paper machine and physically remove the deposits by scraping or some other cleaning means, sometimes multiple times during a shift. This is a common method for maintaining uhle covers for mills producing grades high in resinous pitch. Such cleaning cannot take place during a felt batch cleaning since the batch cleaning chemicals are hazardous and the felt is rotating too closely to the equipment surfaces. It is therefore necessary to extend the time of the shut-down to clean the uhle covers after the felt has been cleaned, causing further losses in production.
Continuous and intermittent felt conditioners have been successful at reducing felt filling and increasing time between batch cleanings. However there is still a need for press section conditioners that not only prevent deposition within felts but also prevent deposition on other equipment surfaces within the press section. This is particularly the case where paper high in resinous or sticky contaminants is being produced. There is a need in the paper industry to provide press section conditioners that passivate equipment surfaces to prevent deposition from forming while also preventing the felt from becoming filled.
Treatments applied to the felt or more particularly to treat equipment surfaces have minimal time (seconds) to passivate the surfaces prior to the introduction of new contaminant as well as new dilution/shower water introduction. This is quite different then the action of treatments applied to act on the contaminants to disperse them, thereby preventing deposition. In such instances significantly more time (several minutes) is available for the action to take place then is available when treating press section equipment.
There still exists a need in the industry to provide a press section conditioner that has improved performance at passivating equipment surfaces such as the uhle box to prevent deposition or inhibits contaminants from filling press felts to enhance the effective life of such felts.